FIG. 3 shows a gas treating apparatus adopted heretofore as a rotor rotating type apparatus for concentrating volatile organic compound (hereinbelow abbreviated to VOC). A honeycomb rotor 31, which is a construction for treating gas in the concentrating apparatus, consists of a well-known honeycomb construction carrying adsorber, which can adsorb VOC with a high efficiency and is rotated continuously with a constant speed by a driving motor not indicated in the figure. On all the peripheral surface of the honeycomb rotor 31 there are disposed an adsorbing zone 32, a desorbing zone 33, which desorbs adsorbed VOC to recover adsorbing capacity of the honeycomb rotor, and a cooling zone 34, which cools the honeycomb rotor heated in the desorbing zone 33, separated from each other.
Consequently the honeycomb rotor 31 is moved by rotation so that it passes through the adsorbing zone 32, desorbing zone 33 and the cooling zone 34 one after another in this order.
Concentration of VOC is effected as follows. Gas to be treated, object to be treated, is introduced into the adsorbing zone 32 and VOC contained in the gas to be treated is adsorbed by adsorber disposed on the honeycomb rotor 31. Therefore the gas to be treated is transformed into clean gas, which is released in the atmosphere. The adsorber, which has adsorbed VOC, in the adsorbing zone 32, which has adsorbed VOC, is moved to the desorbing zone 33, as the honeycomb rotor rotates. In this desorbing zone 33 hot air heated by a heater 35 passes therethrough so that VOC adsorbed by the adsorber disposed on the honeycomb rotor is desorbed by this hot air. The VOC thus desorbed forms concentrated gas and solvent is recovered. FIG. 5 shows a solvent recovering equipment using such a concentrating apparatus as described above.
Since the honeycomb rotor 31 is heated by hot air in the desorbing zone 33, adsorbing capacity of the adsorber disposed on the rotor decreases. In order to recover the adsorbing capacity of the adsorber, the adsorber, which has been in the desorbing zone 33, is moved to the cooling zone 34, as the honeycomb rotor 31 rotates. After having cooled the adsorber by making air in the atmosphere pass through the honeycomb rotor 31 there, the adsorber, which has been in the cooling zone 34, is moved again to the adsorbing zone 32, as the honeycomb rotor 31 rotates.
In this apparatus air effecting desorption and air effecting cooling are used in the form of a continuous flow for the purpose of recovering evacuated heat. That is, atmosphere (air) enters the cooling zone 34 and there receives sensible heat stored in the honeycomb rotor 31 so that hot air flow is formed. This hot air flow is heated by the heater 35 to a temperature required for desorption and introduced into the desorbing zone 33. Thereafter it is taken out as concentrated gas and sent to an after-treatment step.
In the rotor rotating type volatile organic compound concentrating apparatus described above adsorption and desorption of VOC are balanced and required desorption energy is determined, depending on adsorbed amount of VOC and moisture in the gas to be treated. This desorption energy is given by a product of desorption temperature by amount of air flow.
Heretofore the desorbing step was effected by making open air pass through the rotor after having been heated by a heater or a heat exchanger. Desorption gas coming out from the rotor becomes concentrated gas, which is subjected to an after-treatment step, where it is burnt or recovered.
Heating temperature of the rotor is limited by the capacity of the heater or heat exchanger and utility therefor (heat source, electric power, etc.).
In particular, in case where the gas to be adsorbed is wet, more desorption energy is required. When the amount of air flow for desorption is increased in order to increase the desorption energy, the amount of flow of the concentrated gas subjected to the after-treatment step increases also, which lowers the concentration of VOC. For this reason, efficiency of the after-treatment step is worsened, which enlarges the apparatus or increases amount of used utility.
For example, in case where the after-treatment of the concentrated gas is combustion using catalyzer, the concentrated gas is burnt with combustion improver. At this time, if the VOC concentration is low and the amount of air flow is great, the amount of used combustion improver increases.
If the amount of air flow for desorption is decreased, corresponding to the after-treatment, desorption is effected insufficiently so that VOC and moisture remaining in the rotor increase. Then adsorption capacity decreases so that the concentration at the exit of the adsorption rises and the magnification of connection lowers.